Method of treating magnetic material



Dec. 27, 1955 J H. CREDE ET AL 2,728,697

METHOD OF TREATING MAGNETIC MATERIAL Filed NOV. 13, 1952 30 70 50 Flux Densify Gausses INVENTORS Ja/m bf Crede and United States Patent METHOD OF TREATING MAGNETIC MATERIAL John H. Crede and Jack P. Martin, Tarentum, Pa., as-

signors to Allegheny Ludlum Steel Corporation, Brackenridge, Pa., a corporation of Pennsylvania Application November 13, 1952, Serial No. 320,232

6 Claims. (Cl. 148-120) This invention relates to magnetic material and in particular to a method of treating magnetic material.

Heretofore nickel base alloys have been produced in strip form and subjected to a high temperature anneal in an atmosphere of substantially pure dry hydrogen to develop the highest permeability. The nickel base alloy known generally to the trade as Mumetal and having a composition of 75 to 77% nickel, 4 to 5% copper, 1 to 2% chromium and the balance iron with incidental impurities has shown excellent permeability values when annealed at a temperature of1800 to 2150 F. An anneal in excess of 2000 F. and in particular at 2100 F. has consistently developed the highest permeability as measured in the past, such measurement however usually being limited to flux densities above 100 gausses.

Measurements of permeability at values below 100 gausses were seldom attempted because of the inherent difiiculties attending such low level determinations. This -will be appreciated when it is considered that only recently toroidal winding machines have been developed which have eliminated the hours of hand winding of the large number of turns of the pick-up coil on gapless samples as was required prior to the development of such machines.

As commercially practiced heretofore, the nickel base alloy strip was annealedin substantially pure dry hydrogen to prevent oxidation of the strip and to reduce impurities such as carbon, oxygen, sulfur and the like at the high annealing temperature of l800 to 2100 F. and to thereby produce high permeability alloy strip with a very low impurity content. When thus produced, the value of the maximum permeability which occurs in such metal between 2000 and 3000 gausses was usually used as a criterion in determining the efiectiveness of the anneal since this characteristic was the most sensitive to variations in the annealing procedure. Usually the low density permeability values, that is below 100 gausses, was extrapolated to zero flux density from points of higher density.

In many commercial applications and in particular in the electronic field, it is desirable to use a strip material having high permeability at densities of less than 100 gausses and often at densities less than 20 gausses. Accurate measurements of the permeability at such low densities on nickel base strip material treated in accordance with accepted practice for developing maximum permeability at 2000 to 3000 gausses reveals that such commercially accepted practice does not produce high permeability at low flux densities of less than 100 gausses and that, in fact, such practice actually produces a material having a rapid decrease in permeability if measured at less than 30 gausses. The accepted method of extrapolating the permeability curve to zero flux density from points of higher density to establish initial or very low density permeability as has been practiced in the past is therefore erroneous.

An object of this invention is to provide for treating ice nickel base alloy to develop high permeability at flux densities below 100 gausses.

Another object of this invention is to provide for treating an alloy of to 77% nickel, 4 to 5% copper, 1 to 2% chromium and the balance iron with incidental impurities by annealing it at a temperature between 1550 and 1650 F. in a non-oxidizing atmosphere to impart the characteristics of high permeability at flux densities of less than gausses thereto.

Other objects of this invention will become apparent from the following description when taken in conjunction with the accompanying drawing in which the single figure is a graph, the curves of which illustrate the efiect of different annealing temperatures on the permeability values at flux densities of from 1 to 100 gausses on a nickel base alloy.

The nickel base alloy to which this invention is particularly applicable is one having a composition by weight of 75 to 77% nickel, 4 to 5% copper, 1 to 2% chromium and the balance iron with incidental impurities. Carbon, phosphorus, sulphur, manganese, silicon and aluminum may be present in this alloy up to a maximum of 1% each as impurities or residual amounts of addition agents which were added in the refining of the metal for purposes of metallurgical control.

As in usual practice the nickel base alloy is formed as by cold rolling into strip material having a thickness of from .006" to .050", the final cold reduction being in the neighborhood of from 40 to 70%. Such processing of the alloy into cold rolled strip is well-known, being performed by normal mill practice and therefore need not be described further herein.

In accordance with this invention the cold rolled strip of nickel base alloy is preferably annealed at a temperature between l550 and 1650 F. in a non-oxidizing atmosphere for a period of time ranging from 1 to 5 hours. The annealing atmosphere may be any of the non-oxidizing, neutral or reducing atmospheres which are well-known to the trade or may be an atmosphere of substantially pure dry hydrogen such as has been commonly used in the past. By experiment it has been determined that the lower annealing temperature of 1550 F. is necessary in order to obtain the minimum amount of stress relief in the strip material so as to obtain a high initial permeability. On the other hand the optimum temperature of 1650 F. has been determined by examination of many materials, it being found that when the nickel base alloy identified hereinbefore is annealed at a temperature in excess of 1650" F., a large increase in grain size is obtained with a resultant decrease in permeability values at a flux density of less than 100 gausses.

in order to illustrate the benefits obtained by the annealing treatment of this invention, reference may be had to the drawing in which curves 10, 12, 14 16, 18

and 20 illustrate the effects of annealing temperatures of 1400", 1500, 1600", 1700", l800 and 2l00 R, respectively, on the permeability at densities of from 1 to 100 gausses obtained on a strip material having a thickness of .014 and having a composition consisting of 75.57% nickel, 4.5% copper, 1.43% chromium, 035% carbon, .92% manganese, 008% phosphorus, 008% sulphur, 40% silicon, .68% aluminum and the balance iron, and annealed for 4 hours in pure dry hydrogen at each of the temperatures given. As clearly illustrated by curve 14 of the drawing, the anneal of the nickel base alloy strip at a temperature of 1600 F. produces outstanding results insofar as the permeability values at the low flux densities are concerned as compared to the permeability values at low flux densities obtained by annealing the material at temperatures either below or above the range given hereinbefore.

In addition to having an exceptionally high permeability at a low flux density of less than 100 gausses, the treatment of this invention imparts a substantially constant high permeability at low flux densities of less than 100 gausses, it being noted that the permeability obtained after the 1600 F. anneal as shown by curve 14 varies only about from 1 gau'ss up to 100 gausses whereas the permeability obtained on the same material but annealed at 2100 F. as illustrated by curve 20, varies more than 90% when tested in the range between 1 and 100 gausses. Further it is to be noted that the permeability of the nickel base alloy annealed at 1600 F. is exceptionally high when tested in the range of 1 up to 30 gausses as compared to the results obtained by annealing the same material at temperatures below and above the range given hereinbefore as illustrated by curves 10, 12,16, 18 and 20.

While it is not known exactly what phenomena is responsible for the outstanding permeability characteristics at low flux densities that are obtained on the nickel base alloy as annealed at temperatures between 1550 and 1650 F., it is thought that some of the superiority is due to the fact that the temperature employed is sufiiciently high to obtain a high degree of stress relief of the cold rolled strip, but not high enough to result in a substantial grain growth.

In practicing this invention it is not necessary to employ substantially pure dry hydrogen since metallurgical melting of the composition of the alloy has so advanced that the alloy now produced is so substantially free of carbon and sulphur that further purification by the hydrogen atmosphere is no longer necessary in order to obtain high permeability values. Thus, any non-oxidizing atmosphere of which many neutral and reducing gases are well-known may be employed successfully in the method of this invention.

it is to be understood that the method described hereinbefore of annealing the metal known under the trade designation of Mumetal at a temperature of between 1550 F. and 1650 F. is not suitable for imparting high permeability at flux densities materially greater than 100 gausses and in particular at 2000 to 3000 gausses at which the maximum permeability has usually been measured heretofore. As a matter of fact, the annealing treatment of this invention may and usually will yield a material of lower permeability as measured at the high fiux density of 2000 to 3000 gausses than the values obtained with the same material as annealed at temperatures of 1800 to 2150 F. as practiced heretofore. However, for the particular applications which require high permeabilities at low flux densities, the treatment described hereinbefore is outstanding for imparting high substantially constant permeability as measured at flux densities of from 1 up to 100 gausses and in particular from 1 up to 30 gausses.

We claim:

1. The method of treating an alloy of 75 to 77% nickel, 4 to 5% copper, 1 to 2% chromium and the balance iron with incidental impurities, comprising, forming the alloy into strip, and annealing the strip at a temperature within the range of 1550 to 1650 F. to impart the characteristic of high permeability at flux densities below 100 gausses thereto.

2. The method of treating an alloy of 75 to 77% nickel, 4 to 5% copper, 1 to 2% chromium and the balance iron with incidental impurities, comprising, forming the alloy into strip, and annealing the strip in a nonoxidizing atmosphere at a temperature within the range of 1550 to 1650 F. to impart the characteristic of high permeability at flux densities below 100 gausses thereto.

3. The method of treating an alloy of 75 to 77% nickel, 4 to 5% copper, 1 to 2% chromium and the balance iron with incidental impurities, comprising, forming the alloy into a cold rolled strip having a thickness of from .006 to .050 inches by employing a final reduction of from 40 to and annealing the cold rolled strip in a non-oxidizing atmosphere at a temperature within the range of. 1550 to 1650 F. to impart the characteristic of high permeability at flux densities below 100 gasses thereto.

4. The method of treating an alloy of to 77% nickel, 4 to 5% copper, 1 to 2% chromium, and the balance iron with incidental impurities, comprising, forming the alloy into a cold rolled strip having a thickness of from .006 to .050 inch, and annealing the cold roiled strip in a non-oxidizing atmosphere at a temperature of about 1600 F. to impart the characteristic of high permeability at flux densities below gausses thereto.

5. The method of treating an alloy of 75 to 77% nickel, 4 to 5% copperjl to 2% chromium, and the balance iron with incidental impurities, comprising, forming the alloy into a cold rolled strip having a thickness of from .006 to .050 inch, and annealing the cold roiled strip in a hydrogen atmosphere at a temperature of about 1600 F. to impart the characteristic of high permeability at flux densities below 100 gausses thereto.

6. The method of treating an alloy consisting of 75.57% nickel, 4.5% copper, 1.43% chromium, .68% aluminum, .92% manganese, .4% silicon, 035% carbon, 008% phosphorus, 008% sulphur and the balance iron, comprising, forming the alloy into cold rolled strip having a thickness of about 0.14 inch, and annealing the coid rolled strip in hydrogen at a temperature of about 1600* F. to impart the characteristic of high permeability at flux densities below 100 gausses thereto.

2,112,971 Neumann Apr. 5, 1938 

1. THE METHOD OF TREATING AN ALLOY OF 75 TO 77% NICKEL, 4 TO 5% COPPER, 1 TO 2% CHROMIUM AND THE BALANCE IRON WITH INCIDENTAL IMPURITIES, COMPRISING, FORMING THE ALLOY INTO STRIP, AND ANNEALING THE STRIP AT A TEMPERATURE WITHIN THE RANGE OF 155* TO 1650* F. TO IMPART THE CHARACTERISTIC OF HIGH PERMEABILITY AT FLUX DENSITIES BELOW 100 GAUSSES THERETO. 